Lorenzo Insigne thought he had put Napoli ahead after two minutes but Hirving Lozano was ruled offside in the build-up. After that, it was a lacklustre display from Napoli which further angered their supporters.
Napoli defender Kalidou Koulibaly prevented it from being worse for the hosts with a miraculous clearance off the line from Andrea Pinamonti's point-blank shot in the second-half.
Napoli, runners-up in three of the last four seasons, stayed seventh on 19 points, 12 behind leaders Inter Milan. (Reporting by Brian Homewood, editing by Ed Osmond)